It is desirable in many olefin polymerization processes, particularly a slurry phase or gas phase process, to use a supported catalyst system. A particularly useful catalyst system typically includes a metallocene catalyst and an alumoxane supported on a carrier, such as silica. (See, e.g., U.S. Pat. Nos. 5,643,847, 5,972,823, 6,143,686, 6,228,795, and 6,368,999).
While all these supported catalyst systems are useful, it would be desirable to have an improved catalyst system or process which in producing polymers resists fouling the reactor. Particularly in a slurry or gas phase polymerization processes, there is a tendency for reactor operation problems during polymerization. During a typical polymerization process, polymer within the reactor often accumulates and clings or sticks to the walls of a reactor. After a relatively short period of time during polymerization, polymer foulant formed from the aggregation of polymers begins to appear in the reactor, and this foulant can break free and plug product discharge systems forcing shutdown of the reactor.
The accumulation of polymer particles on the reactor surfaces and internals of the reactor and cooling system results in many problems. Of particular importance is the problem of poor heat transfer during the polymerization process. The foulant can trap heat along the reactor wall by the retardation of the normal convective forces that dissipate heat in the reactor.
Therefore, it would be highly desirable to have an improved polymerization catalyst system or polymerization process that would significantly enhance reactor operability while minimizing reactor fouling.
The prior art contains a number of different teachings regarding the minimization of fouling and sheeting in commercial scale, continuous olefin polymerization processes. For example, U.S. Pat. No. 6,022,935 to Fisher et al. discloses the preparation of polymers of alk-1-enes in the presence of a supported metallocene catalyst system and an antistatic agent. The preparation is performed from −50 to 300° C. and from 0.5 to 3000 bar in the presence of the catalyst system. The antistatic agent is preferably Stadis 450. It is used as a solution, preferably from 1 to 50, or particularly preferably from 5 to 25, % by weight of the solution, based on the mass of the supported catalyst used (carrier, metallocene complex and compound forming metallocenium ions). However, Fisher teaches that the required amounts of antistatic agent may vary within wide ranges of polymerization conditions and antistatic concentration depending on the type of antistatic agent used.
U.S. Patent Application Publication No. 2002/0091208 A1 to Benazouzz et al. discloses a process for the gas-phase (co-)polymerization of olefins in a fluidised bed reactor using a metallocene catalyst in the presence of an antistatic agent. In particular, Benazouzz discloses a process for the gas-phase (co-)polymerization of olefins in a fluidised bed reactor using a metallocene catalyst in the presence of an antistatic agent characterized in that the antistatic agent comprises at least one of the components selected from: (1) a polysulfone copolymer, (2) a polymeric polyamine, and (3) an oil-soluble sulfonic acid. Benazouzz teaches a wide variety of polymer products. Among them are linear low density polyethylene (LLDPE) based on copolymers of ethylene with but-1-ene, 4-methylpent-1-ene or hex-1-ene and high density polyethylene (HDPE) which can be for example copolymers of ethylene with a small portion of higher alpha olefin, for example, but-1-ene, pent-1-ene, hex-1-ene or 4-methylpent-1-ene. Additionally, it teaches that when liquid condenses out of the recycle gaseous stream, it can be a condensable monomer, e.g., but-1-ene, hex-1-ene, 4-methylpent-1-ene or octene used as a comonomer, and/or an optional inert condensable liquid, e.g. inert hydrocarbon(s), such as C4–C8 alkane(s) or cycloalkane(s), particularly butane, pentane or hexane.
Other background references include U.S. Pat. Nos. 6,562,924, 5,026,795, 4,012,574, WO 01/90239, WO 01/83498, WO 00/68274, WO 00/05277, WO 99/67307, WO 96/11960, EP 0 811 638 A, and EP 0 453 116 A.
However, past attempts have failed to adequately teach optimized polymerization processes for propylene polymer production that enhance reactor operability while minimizing reactor fouling. The invention solves these problems by using low levels of antistatic agent during polymerization while maintaining optimized process conditions. Additionally, the invention further solves these problems by contacting the antistatic agent with a scavenger before contacting monomers and/or the catalyst system.